The present invention relates to an automatic reflow soldering apparatus for soldering a printed circuit board having a plurality of electronic elements mounted thereon to be soldered thereto by use of a heated inert gas, and more particularly relates to such apparatus which is easily responsive to mild and rapid flows of the inert gas in the apparatus to prevent the inert gas from flowing out of the apparatus to thereby remarkably reduce the wasteful consumption of the inert gas.
According to the conventional automatic soldering apparatus, the printed circuit board has been soldered by a molten solder in the air containing much oxygen. In this method, the solder and the electronic elements to be soldered to the printed circuit board are exposed to the oxygen in a high temperature and therefore the soldered portions and the molten solder are easily oxidized. As the result, it has been very difficult to attain a sufficient soldering effect. It has been often required to take a troublesome work such as to remove the residues from the soldered printed circuit board.
Recently the electronic elements have been very small and the leads accordingly have been very fine and therefore a small defect such as the oxidization of soldered portions will decrease the reliability of the electronic goods.
For the purpose of solving such a problem, there has been proposed an automatic soldering apparatus in which the solder and the printed circuit board having the electronic elements mounted thereon are heated up to a high temperature in an inert gas such as the nitrogen gas so that the soldered portions and the molten solder will not be oxidized.
The inert gas such as the nitrogen gas to be employed in the apparatus of this kind is expensive, and it is required to tightly close up the apparatus so as to reduce the leaks of the inert gas from the apparatus to a minimum.
On the other hand, it is inevitable that such apparatus will transport the printed circuit board thereinto and transport the soldered printed circuit board out of apparatus. Regarding the automatic soldering apparatus, it has been very difficult to prevent the inert gas from flowing out of the apparatus at the openings through which the printed circuit board is transported into and out, of the apparatus. Actually it has been a observed that a higher amount of inert gas leaks from the apparatus uneconomically.
In short, it is generally known that the automatic reflow soldering apparatus, by way of example, is under control such that the temperature is progressively higher in a plurality of chambers, that is, from a preheating chamber to a reflow soldering chamber. Since the gas density is lighter in a higher temperature and is heavier in a lower temperature, the inert gas will constantly flow from the place of a lower temperature to the place of higher temperature. This flow of inert gas due to the difference of temperatures will cause the gas leak at the exit of the printed circuit board in the soldering apparatus.
Further the gas leak at the exit will cause the outer air to flow into the apparatus at the entrance of the printed circuit board in the apparatus resulting in a dilution of the inert gas within the apparatus and lowering the oxidization resistant effect. It is therefore apparent that a desired soldering effect can not be attained.
The flow of the inert gas from the chamber of a lower temperature to the chamber of a higher temperature is not always constant. The flow is more or less varied due to the soldering conditions in the automatic soldering apparatus. Such a slight change of the flow will cause the inert gas to leak from the entrance or the exit of the apparatus.
As is generally known, the automatic soldering apparatus has a ventilator provided in each of the preheating chambers and also in the reflow soldering chamber for the purpose of circulating the highly heated inert gas from up to down so that the inert gas may be blown to the upper surface of the printed circuit board to be soldered. The inert gas blown against the upper surface of the printed circuit board will partly flow in the opposite directions as the printed circuit board is transported. This further results in increasing the leaks of the inert gas.
Further just when the printed circuit board is transported at the entrance into the blown and circulated inert gas in the preheating chamber, a part of the inert gas will rapidly flow toward the entrance along the upper surface of the printed circuit board, and flows out of the entrance and is thus consumed. The same consumption phenomenon is caused when the print circuit board is transported out of a cooling chamber. Namely a part of the inert gas will rapidly flow out of the exit of the soldering apparatus.
A device has been proposed to increase the supply the inert gas to increase the pressure in the soldering apparatus so as to prevent the air from flowing into the apparatus. Such a method will require a considerable amount of inert gas and is not economical. In fact, such a method may be effective to maintain a constant flow of inert gas in the direction from the chamber of a lower temperature to the chamber of a higher temperature, but is not enough to control the rapid change of the inert gas flow.
Actually from the economical reason, a device is required to adjust the supplied amount of inert gas so as to reduce the consumption of inert gas to a minimum which may flow out of the soldering apparatus. However the device will not sufficiently respond to the influence of a strong outer air flow into the soldering apparatus at the entrance or the exit thereof through which the printed circuit board is transported into or out of the apparatus. This will adversely influence the soldering effect of the printed circuit board.
The present invention has been provided to eliminate the defects and disadvantages of the prior art. It is the principal object of the invention to provide a duct which has a front end opening and a rear end opening which are connected to a front station and a rear station of a soldering apparatus respectively through which the printed circuit board is transported into and out of the soldering apparatus, the areas of the front and rear openings of the duct being larger than the areas of the entrance and the exit of the soldering apparatus, so that the pressures within the front and rear stations may be constantly maintained equal to thereby prevent unnecessary flows of the inert gas and to thereby prevent the inert gas from flowing out of the entrance and the exit of the soldering apparatus.
It is another object of the invention to provide a first inert gas supplying device on the side of the front end of the duct and a second inert gas supplying device on the side of the rear end of the duct so as to control the amount of inert gas to be supplied into the front and rear stations of the soldering apparatus in response to the change of air flows therein, to thereby control the flow of inert gas in the soldering apparatus and thus to remarkably reduce the consumption of inert gas.
It is still another object of the invention to supply the inert gas through the duct into the front station or into the rear station when the inert gas in the preheating chamber flows rapidly to the entrance of the printed circuit board or when the inert gas in the cooling chamber flows rapidly to the exit of the print circuit board to thereby control the rapid flows of inert gas in the soldering apparatus and to thereby prevent the inert gas from flowing out of the soldering apparatus at the entrance and the exit thereof and further to thereby reduce the change of inert gas density to a minimum in the soldering apparatus, and thus to maintain the effective soldering performance of the apparatus and simultaneously to reduce the consumption of the inert gas.